1. Field of the Invention
The invention relates to a method for readout of rotation rate with a passive optical ring resonator of the type in which light from a tuned coherent light source passes through a mode filter, then is split into two partial light beams radiated in opposite directions into a ring resonator, and in which the light components of both directions of circulation are then coupled out.
2. Description of the Prior Art
In addition to the active resonator (laser gyro) and the Sagnac interferometer, the passive resonator is a suitable optical means for measuring rotation rate by the Sagnac effect (G. Sagnac: C. R. Acad. Sci. Paris 95, 708 (1913)).
Approximately 10 years ago, S. Ezekiel and S. R. Balsamo investigated the passive ring resonator at the Massachusetts Institute of Technology for suitability as a rotation rate sensor. An initial fundamental technical solution is disclosed in U.S. patent Ser. No. 4,135,822. Initial experimental results were published a short time later (Appl. Phys. Lett., Vol. 30, p. 478 (1977)). Constant further development of the experimental model then led to a rotation sensor having inertial accuracy under laboratory conditions (Opt. Lett, Vol. 6, p. 569 (1981)).
Although the resonators in accordance with the experimental models were constructed using reflector technology, the possibility of a fiber resonator was considered at an early stage. See, for example, U.S. patent Ser. No. 4,135,822. However, an early fiber construction encountered technical problems. With the addition of a commercial coupler of high quality, however, it became possible to construct a resonator that allowed successful measurements to be carried out (R. E. Meyer et al., "Passive Fiberoptic Ring Resonator for Rotation Sensing", Preprint, MIT (1983)).
Parallel research efforts in the United States, particularly that of the E. L. Ginzton Laboratory at Stanford University, led to the development of a low-loss fiber directional coupler (Electron. Lett., Vol. 16, p. 260 (1980)). By employing such couplers, it became possible to produce resonators having a fineness of 60 to 90 (cf. L. F. Stokes et al., Opt. Lett., Vol. 7, p. 188 (1982)). Recently, a fiber ring resonator of fineness exceeding 600 was reported (M. Kemmler, K. Kempf, W. Schroder, Technical Digest, Optical Fiber Sensors, p. 85, San Diego). Experimental investigations into suitability as a rotation sensor were reported (cf. G L. Report No. 3620, E. L. Ginzton Lab, Stanford University, September 1983).
The development of passive resonators in integrated optics has also become known. See, for example, U.S. patent Ser. No. 4,326,803 and the Northrop company publication by A. Lawrence, "The MicroOptical Gyro", Northrop Precision Products Division, August 1983).
A resonator constructed of reflector technology has the disadvantage for a rotation rate sensor that the strict maintenance of the axial TEMoo mode in the resonator is difficult under unfavorable environmental conditions. A fiber resonator construction, to the contrary, has the advantage of a lesser temperature gradient sensitivity in comparison to a Sagnac interferometer due to the substantially shorter fiber length required (cf. D. M. Shupea, Appl. Opt. Vol. 20, p. 186 (1981)). However, a fiber ring can carry two polarization eigenstates. (cf. B. Lamouroux et al., Opt. Lett., Vol. 7, p. 391 (1982)). The coupling of such states as a result of environmental influences can lead to null-point fluctuations in the output channel.
Additionally, only single-mode He-Ne lasers have been employed to date as light sources. The backscatter occurring in the fiber resonator is a substantial cause of interferences that disturb the useful signal. Application of one or more longer-wave-length coherent light sources theoretically provides improvement as the Rayleigh scattering is inversely proportional to the fourth power of the wavelength of the light. However, to date the obvious application of a longer-wavelength semiconductor laser has foundered as its spectral width is excessive for a good fiber resonator. A significant reduction of the spectral width of a semiconductor laser may be achieved by application of an external resonator (see S. Saito and Y. Yamamoto, Electr. Lett., Vol. 17, p. 325 (1981); M. W. Fleming and A. Mooradian, IEEE J. Quant. Electr. QE, Vol. 17, p. 44 (1981)). Such a light source can be attained by adding one or more dispersive elements, grating and/or mirrors to the semiconductor laser or by coating the semi-conductor laser in such a way that the quality of the optical resonator is increased. The construction of an external resonator of fiber technology has been proposed. IEEE Transactions on Microwave Theory and Techniques, MTT 30, No. 10, 1700 (1982)).
The problem of undesired low-frequency interferences in the useful signal due to the mixing of a signal wave with the backscattered component of the oppositely directed wave invariably occurs in circumstances in which the two oppositely directed light waves occupy the same longitudinal resonator mode. A known remedy is phase modulation in the arrival optical path to the resonator (Sanders et al., Opt. Lett., Vol. 6, p. 569 (1981)).
A further possibility is the application of frequency-shifting elements such as Bragg cells. In such a way, the interference signal between forward-scattered and backscattered light appears outside the detection bandwidth.
All hitherto-known resonator readout methods have utilized only the intensity information of the ring resonator to evaluate the rate of rotation. In this regard, the following difficulty does, however, arise: the cross-coupling of the two polarization eigenstates of the ring resonator leads to instabilities of the null point of the gyro output signal. A reciprocal configuration must be selected to remedy this requiring the application of two mode filters (spatial filter and polarization together).